The invention concerns the preparation of novel structured foams on the basis of polycyanurate resin. The foams are characterized by an especially minimal specific weight, high flame retardation and excellent mechanical properties. They are therefore suitable materials for lightweight construction applications in transportation technology, in particular in aviation. The method is based on coupling of the curing process with chemical generation of the foaming agent required for foaming for which purpose in the present invention carbon dioxide is utilized.
The demand in regard to permanent weight reduction, thermal insulation and sound damping as well as increased safety requirements in the transportation sector can be met by increased utilization of high quality structured foams. In addition to excellent mechanical properties, these structural components must also be highly fire-resistant. The requirements imposed in civil aviation (interior furnishings) with respect to fire behavior include minimal flammability, reduced heat release rate, low smoke density as well as minimal toxicity of the gases generated in a fire.
Analog requirements are imposed in the construction of rail vehicles.
Polyurethanes are reactive resin foam materials having the broadest application spectrum because of their excellent price/performance ratio, variability, and excellent processing properties. However, for advanced structural applications in the transportation sector, the inherent fire-relevant properties of the polyurethane hard foams do not suffice. Foams on the basis of high-temperature thermoplastic materials such as PEEK and PEI are only used in niche applications—also because of their high costs—and can be produced only as blocks or as extruded sheets.
Phenolic resin foams are used in mining for stabilizing cavities in rock because of the possibility of foaming multi-dimensional structures in a simple way and because of the excellent inherent flame retardation. As a result of the small closed cell structure, phenolic resin foams belong to foams with the least thermal conductivity and exhibit therefore also excellent thermal insulation properties. Phenolic resin foams are however brittle hard foams that are prone to sand and to form cracks and therefore are not mechanically loadable to a sufficient degree for use in structural applications.
As has been demonstrated many times, cyanate resins have a set of excellent properties combining high inherent flame retardation with excellent thermal and mechanical properties. Polymers of polyfunctional cyanic acid esters of the general formula R(OCN)n are increasingly used in various applications in particular as laminate resins and adhesives in high-tech areas. They are characterized in particular by a high glass transition temperatures, high decomposition temperatures, low flammability already without addition of flame-retardant additives, high ductility in comparison to other high Tg duromers, low dielectric losses, excellent adhesion to various substrate materials, high chemical resistance, and minimum corrosion potential. Also, they are a material that can be recycled. Therefore, cyanate resins are promising base materials for a variety of applications.
The curing reaction of polycyanates (polycyclotrimerization to polycyanurates) happens relatively slowly for very pure monomers. Contaminants from the monomer synthesis, for example, residual phenols or traces of water, catalyze the curing reaction that is exothermic and therefore easily runs out of control. In order to avoid this, cyanate starting compounds are employed that are as pure as possible and the curing reaction is started in targeted way by addition of suitable catalyst. For this purpose, in particular metal acetyl acetonate, zinc salts, different organometallic compounds as well as phenols and compounds with NH groups such as primary and secondary amines are used. The latter react smoothly with cyanates, as has been known since the 1960s and disclosed, for example, in U.S. 2003/0176616 A1 and the counterpart U.S. Pat. No. 6,822,067 B1. They act therefore not only as catalysts but also as reaction partners for the cyanates. With more than 10 mole % of the amine compound, relative to the cyanate groups, resins are obtained that in addition to the triazine group (A) also contain the following structural elements B, C, and/or D.

It is desirable to provide also foamed polycyanurates in the form of structural foams because they can fulfill the highest FST requirements (FST=flame, smoke, toxicity). A further advantage is that the ductility of the resin matrix in wide ranges can be tailored to the requirements of the application, respectively. Furthermore, on the basis of cyanate resins foams with excellent thermal insulation properties should be producible because the adjustable processing viscosities are similar to those of phenolic resins and therefore similar cell structures should thus result. Also, cyanate resins, as a result of the chemical mechanism, can be foamed in an environmentally friendly way and they can be recycled at the end of their service life with already known and patented methods.
In the literature there are a few proposals for producing cyanate foams. For example, in EP 0457692 B1 and U.S. Pat. No. 5,077,319 methods are disclosed that are based on foaming mixtures of cyanate resin precursors (e.g. 2,2-bis-(4-cyanatophenyl)methane) and a thermoplastic material such as a polysulfone, polyether sulfone, polyimide, or polyacrylate or the like. Foaming is achieved primarily by a chemical foaming agent (for example, azodicarbonamide). This method is however extremely difficult to handle: the decomposition of this agent with formation of gas is generally exothermic as is the cyanate polymerization; therefore, the latter is hardly controllable. Moreover, the curing temperatures are very high. Optionally, physical foaming agents (e.g. methyl isobutyl ketone) are recommended. Of course, they are unacceptable with regard to environmental concerns and with regard to possible auto ignition. Films of the starting materials are foamed at high temperatures (>200° C.). The materials are also suitable for filling honeycombs (U.S. Pat. No. 5,338,594). The examples of application indicate that the method is suitable only for high contents of thermoplastic material.
According to U.S. Pat. No. 6,506,808 cyanurate prepolymers are loaded at high pressure with CO2. The decompression then leads to foaming. The decompression process is carried out again at high temperatures (170-180° C.). Moreover, only relatively dense foams are obtained (ρ≈1.2 g/cm3).